Chapter 13 : Surface Areas And Volumes

13.1 Introduction

Wherever we look, usually we see solids. So far, in all our study, we have been dealing
with figures that can be easily drawn on our notebooks or blackboards. These are
called plane figures. We have understood what rectangles, squares and circles are,
what we mean by their perimeters and areas, and how we can find them. We have
learnt these in earlier classes. It would be interesting to see what happens if we cut
out many of these plane figures of the same shape and size from cardboard sheet and
stack them up in a vertical pile. By this process, we shall obtain some solid figures
(briefly called solids) such as a cuboid, a cylinder, etc. In the earlier classes, you have
also learnt to find the surface areas and volumes of cuboids, cubes and cylinders. We
shall now learn to find the surface areas and volumes of cuboids and cylinders in
details and extend this study to some other solids such as cones and spheres.

13.2 Surface Area Of A Cuboid And A Cube

13.3 Surface Area Of A Right Circular Cylinder

If we take a number of circular sheets of paper and stack them up as we stacked up
rectangular sheets earlier, what would we get (see Fig. 13.6)?

Here, if the stack is kept vertically up, we get what is called a right circular
cylinder, since it has been kept at right angles to the base, and the base is circular. Let
us see what kind of cylinder is not a right circular cylinder.

13.4 Surface Area Of A Right Circular Cone

So far, we have been generating solids by stacking up congruent figures. Incidentally,
such figures are called prisms. Now let us look at another kind of solid which is not a
prism. (These kinds of solids are called pyramids). Let us see how we can generate
them.

Activity : Cut out a right-angled triangle ABC right angled at B. Paste a long thick
string along one of the perpendicular sides say AB of the triangle [see Fig. 13.13(a)].
Hold the string with your hands on either sides of the triangle and rotate the triangle about the string a number of times. What happens? Do you recognize the shape that
the triangle is forming as it rotates around the string [see Fig. 13.13(b)]? Does it
remind you of the time you had eaten an ice-cream heaped into a container of that
shape [see Fig. 13.13 (c) and (d)]?

13.5 Surface Area Of A Sphere

What is a sphere? Is it the same as a circle? Can you draw a circle on a paper? Yes,
you can, because a circle is a plane closed figure whose every point lies at a constant
distance (called radius) from a fixed point, which is called the centre of the circle.
Now if you paste a string along a diameter of a circular disc and rotate it as you had
rotated the triangle in the previous section, you see a new solid (see Fig 13.18). What
does it resemble? A ball? Yes. It is called a sphere.

13.6 Volume Of A Cuboid

You have already learnt about volumes of certain figures (objects) in earlier classes.
Recall that solid objects occupy space. The measure of this occupied space is called
the Volume of the object.

Note : If an object is solid, then the space occupied by such an object is measured,
and is termed the Volume of the object. On the other hand, if the object is hollow, then
interior is empty, and can be filled with air, or some liquid that will take the shape of its
container. In this case, the volume of the substance that can fill the interior is called the
capacity of the container. In short, the volume of an object is the measure of the
space it occupies, and the capacity of an object is the volume of substance its interior
can accommodate. Hence, the unit of measurement of either of the two is cubic unit.

13.7 Volume Of A Cylinder

Just as a cuboid is built up with rectangles of the same size, we have seen that a right
circular cylinder can be built up using circles of the same size. So, using the same
argument as for a cuboid, we can see that the volume of a cylinder can be obtained

13.8 Volume Of A Right Circular Cone

In Fig 13.28, can you see that there is a right circular
cylinder and a right circular cone of the same base
radius and the same height?

Try to Activity :make a hollow cylinder and a hollow cone like this with the same
base radius and the same height (see Fig. 13.28). Then, we can try out an experiment
that will help us, to see practically what the volume of a right circular cone would be!

13.9 Volume Of A Sphere

Now, let us see how to go about measuring the volume of a sphere. First, take two or
three spheres of different radii, and a container big enough to be able to put each of
the spheres into it, one at a time. Also, take a large trough in which you can place the
container. Then, fill the container up to the brim with water [see Fig. 13.30(a)].

Now, carefully place one of the spheres in the container. Some of the water from
the container will over flow into the trough in which it is kept [see Fig. 13.30(b)].
Carefully pour out the water from the trough into a measuring cylinder (i.e., a graduated
cylindrical jar) and measure the water over flowed [see Fig. 13.30(c)]. Suppose the
radius of the immersed sphere is r (you can find the radius by measuring the diameter
of the sphere).